Air pollution control system and air pollution control method

ABSTRACT

An air pollution control system  10 A includes a boiler 11 that burns fuel, an air heater  13  that recovers heat of flue gas  17  from the boiler  11 , and a desulfurizer  15  that reduces sulfur oxides contained in the flue gas  17  after heat recovery by an absorbent, and waste-water supplying units P 0  to P 5  that supply desulfurized waste water  28  discharged from the desulfurizer  15  to at least one of a path for supplying fuel to the boiler  11 , inside of a furnace of the boiler  11 , and the inside of a flue gas duct between the boiler  11  and the air heater  13  are installed. With this configuration, an amount of desulfurized waste water to be returned into the flue gas duct per unit time can be increased as compared to conventional systems, without increasing the size of the entire air pollution control system.

FIELD

The present invention relates to an air pollution control system and anair pollution control method for treating flue gas discharged from aboiler.

BACKGROUND

Conventionally, there has been known an air pollution control system fortreating flue gas discharged from a boiler installed in a thermal powergeneration plant or the like. The air pollution control system includesNO_(x) removal equipment that removes nitrogen oxides from flue gasdischarged from a boiler, an air heater that recovers heat of flue gashaving passed through the NO_(x) removal equipment, a precipitator thatreduces dust in the flue gas after heat recovery, and a desulfurizerthat reduces sulfur oxides in the flue gas after dust reduction. As thedesulfurizer, a wet desulfurizer that reduces sulfur oxides in flue gasby bringing a limestone absorbent into gas-liquid contact with flue gashas been generally used.

Waste water discharged from a wet desulfurizer (hereinafter,“desulfurized waste water”) contains various types of harmfulsubstances, for example, ions such as chlorine ion and ammonium ion andmercury in large amount. Therefore, these harmful substances need to beremoved from the desulfurized waste water before the desulfurized wastewater is discharged to outside of the system. However, a removingprocess of these various types of harmful substances contained in thedesulfurized waste water is complicated, and treatment cost is high.Therefore, to reduce the treatment cost of the desulfurized waste water,there has been proposed a method of reusing the desulfurized waste waterin the system without discharging it to the outside of the system. Forexample, Patent Literature 1 discloses an air pollution control systemin which a device that atomizes and gasifies desulfurized waste water isseparately installed, branched from a flue gas duct of a main line thatconnects NO_(x) removal equipment, an air heater, a precipitator, and adesulfurizer, and after a part of flue gas is introduced from the fluegas duct of the main line into the device, and desulfurized waste wateris atomized into flue gas in the device and evaporated to precipitateharmful substances, the flue gas is returned to the flue gas duct of themain line.

Citation List Patent Literature

Patent Literature 1: Japanese Patent Application Laid-open No. H9-313881

SUMMARY Technical Problem

However, in the air pollution control system according to PatentLiterature 1, because a device that atomizes and gasifies desulfurizedwaste water is separately installed, branched from the flue gas duct ofthe main line, and after a part of flue gas is introduced from the fluegas duct of the main line into the device, and desulfurized waste wateris atomized into flue gas in the device and evaporated, the flue gas isreturned to the flue gas duct of the main line, a device for evaporatingdesulfurized waste water needs to be provided separately. Therefore,there is a problem that the size of the entire air pollution controlsystem increases.

Generally, when the treating amount of flue gas increases, the amount ofdesulfurized waste water also increases in proportion thereto. However,in the air pollution control system according to Patent Literature 1,because an amount of gas that can be returned to the flue gas duct islimited by the treating capacity of the atomizing device, a large amountof desulfurized waste water cannot be treated in unit time. As a result,the treating amount of flue gas is reduced.

Therefore, it has been desired to increase the amount of desulfurizedwaste water to be returned to the flue gas duct per unit time ascompared to conventional systems, without increasing the size of theentire air pollution control system.

The present invention has been achieved to solve the above problems, andan object of the present invention is to provide an air pollutioncontrol system and an air pollution control method capable of increasingthe amount of desulfurized waste water to be returned to a flue gas ductper unit time as compared to conventional systems, without increasingthe size of the entire air pollution control system.

Solution to Problem

According to an aspect of the present invention, an air pollutioncontrol system includes: a boiler that burns fuel; an air heater thatrecovers heat of flue gas from the boiler; a desulfurizer that reducessulfur oxides contained in flue gas after heat recovery by an absorbent;a waste-water supplying unit that supplies desulfurized waste waterdischarged from the desulfurizer to at least one of a path for supplyingfuel to the boiler, inside of a furnace of the boiler, and inside of aflue between the boiler and the air heater.

Advantageously, in the air pollution control system, a NO_(x) removalequipment that removes nitrogen oxides in flue gas from the boiler isprovided on an upstream side of the air heater, and the waste-watersupplying unit is provided at least at one position between the boilerand the NO_(x) removal equipment or between the NO_(x) removal equipmentand the air heater.

Advantageously, in the air pollution control system, a bypass pipe isprovided at least at one position parallel to the NOx removal equipmentor parallel to the air heater, and the waste-water supplying unit isprovided in the bypass pipe.

According to another aspect of the present invention, an air pollutioncontrol system includes: a boiler that burns fuel; an air heater thatrecovers heat of flue gas from the boiler; a desulfurizer that reducessulfur oxides contained in flue gas after heat recovery by an absorbent;a waste-water treating unit that removes harmful substances fromdesulfurized waste water discharged from the desulfurizer; and awaste-water supplying unit installed at least at one position in a pathfor supplying fuel to the boiler, inside of a furnace of the boiler, orinside of a flue between the boiler and the air heater to atomizetreated waste water treated by the waste-water treating unit.

Advantageously, in the air pollution control system, a NO_(x) removalequipment that removes nitrogen oxides in flue gas from the boiler isprovided on an upstream side of the air heater, and the waste-watersupplying unit is provided at least at one position between the boilerand the NO_(x) removal equipment or between the NO_(x) removal equipmentand the air heater.

Advantageously, in the air pollution control system, a bypass pipe isprovided at least at one position parallel to the NOx removal equipmentor parallel to the air heater, and the waste-water supplying unit isprovided in the bypass pipe.

Advantageously, in the air pollution control system, the waste-watertreating unit includes a solid-liquid separating device that separatesdesulfurized waste water discharged from the desulfurizer into a solidand a liquid.

Advantageously, in the air pollution control system, the waste-watertreating unit includes a mercury removing device that removes mercurycontained in desulfurized waste water discharged from the desulfurizer.

Advantageously, in the air pollution control system, the waste-watertreating unit includes a halogen-ion removing device that removeshalogen ions contained in desulfurized waste water discharged from thedesulfurizer.

According to still another aspect of the present invention, in an airpollution control method in which after heat of flue gas from a boilerthat burns fuel is recovered by an air heater, a desulfurizer reducessulfur oxides contained in flue gas after heat recovery by an absorbent,desulfurized waste water discharged from the desulfurizer is supplied toat least one of a path for supplying fuel to the boiler, inside of afurnace of the boiler, and inside of a flue gas duct between the boilerand the air heater.

Advantageously, in the air pollution control method, a NO_(x) removalequipment that removes nitrogen oxides in flue gas from the boiler isprovided on an upstream side of the air heater, and the desulfurizedwaste water is supplied to at least one position between the boiler andthe NO_(x) removal equipment or between the NO_(x) removal equipment andthe air heater.

Advantageously, in the air pollution control method, a bypass pipe isprovided at least at one position parallel to the NO_(x) removalequipment or parallel to the air heater, and the desulfurized wastewater is supplied to inside of the bypass pipe.

According to still another aspect of the present invention, in an airpollution control method in which after heat of flue gas from a boilerthat burns fuel is recovered by an air heater, a desulfurizer reducessulfur oxides contained in flue gas after heat recovery by an absorbent,after waste water treatment is performed for removing harmful substancesin desulfurized waste water discharged from the desulfurizer, treatedwaste water treated in the waste water treatment is supplied to at leastone of a path for supplying fuel to the boiler, inside of a furnace ofthe boiler, and inside of a flue gas duct between the boiler and the airheater.

Advantageously, in the air pollution control method, a NO_(x) removalequipment that removes nitrogen oxides in flue gas from the boiler isprovided on an upstream side of the air heater, and the treated wastewater is supplied to at least one position between the boiler and theNO_(x) removal equipment or between the NO_(x) removal equipment and theair heater.

Advantageously, in the air pollution control method, a bypass pipe isprovided at least at one position parallel to the NO_(x) removalequipment or parallel to the air heater, and the treated waste water issupplied to inside of the bypass pipe.

Advantageously, in the air pollution control method, the waste watertreatment includes a solid-liquid separating step of separatingdesulfurized waste water discharged from the desulfurizer into a solidand a liquid.

Advantageously, in the air pollution control method, the waste watertreatment includes a mercury removing step of removing mercury containedin desulfurized waste water discharged from the desulfurizer.

Advantageously, in the air pollution control method, the waste watertreatment includes a halogen-ion removing step of removing halogen ionscontained in desulfurized waste water discharged from the desulfurizer.

Advantageous Effects of Invention

According to the air pollution control system and the air pollutioncontrol method of the present invention, because desulfurized wastewater is directly atomized to at least one of the path for supplyingfuel to the boiler, the inside of a furnace of the boiler, and the fluegas duct between the boiler and the air heater, a device that evaporatesand gasifies the desulfurized waste water does not need to be providedseparately like in conventional systems. As a result, the amount ofdesulfurized waste water from the desulfurizer to be discharged tooutside of the system can be decreased without increasing the size ofthe air pollution control system.

Further, because flue gas in the boiler and in the flue gas duct betweenthe boiler and the air heater is high-temperature gas before heatrecovery by the air heater, a large amount of desulfurized waste watercan be evaporated in the boiler and in the flue gas duct. Therefore, theamount of waste water to be returned to the flue gas duct per unit timecan be increased as compared to conventional systems. As a result, theamount of desulfurized waste water to be treated can be increased ascompared to conventional systems, and as a result, the treating amountof flue gas per unit time can be increased.

According to the air pollution control system and the air pollutioncontrol method of the present invention, waste water treatment forremoving harmful substances from desulfurized waste water dischargedfrom the desulfurizer is performed, and treated waste water havingundergone the waste water treatment is directly atomized to at least oneof the path for supplying fuel to the boiler, the inside of a furnace ofthe boiler, and the flue gas duct between the boiler and the air heater.As a result, an increase in concentration of harmful substances in fluegas inside the flue gas duct can be prevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic configuration diagram of an air pollution controlsystem according to a first embodiment.

FIG. 2 is a schematic configuration diagram of an air pollution controlsystem according to a second embodiment.

FIG. 3 is a schematic configuration diagram of an air pollution controlsystem according to a third embodiment.

FIG. 4 is a schematic configuration diagram of an air pollution controlsystem according to a fourth embodiment.

FIG. 5 is a schematic configuration diagram of an air pollution controlsystem according to a fifth embodiment.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of the present invention will be explained belowin detail with reference to the accompanying drawings. The presentinvention is not limited to the embodiments. In addition, constituentelements in the following embodiments include those that can be easilyassumed by persons skilled in the art or that are substantiallyequivalent.

First Embodiment

FIG. 1 is a schematic configuration diagram of an air pollution controlsystem according to a first embodiment. An air pollution control system10A in FIG. 1 removes harmful substances such as nitrogen oxides(NO_(x)), sulfur oxides (SO_(x)), and mercury (Hg) from flue gas 17discharged from a boiler 11 such as a coal combustion boiler that usescoals as a fuel or a heavy-oil combustion boiler that uses heavy oil asa fuel.

The air pollution control system 10A includes NO_(x) removal equipment12 that removes nitrogen oxides in the flue gas 17 from the boiler 11,an air heater 13 that recovers heat of the flue gas 17 having passedthrough the NO_(x) removal equipment 12, a precipitator 14 that reducesdust in the flue gas 17 after heat recovery, a desulfurizer 15 thatreduces sulfur oxides in the flue gas 17 after dust reduction accordingto a wet method, and a waste-water atomizing device 16 that suppliesdesulfurized waste water 28 discharged from the desulfurizer 15 to atleast one of a path for supplying fuel F to the boiler 11, the inside ofa furnace of the boiler 11, and the inside of the flue gas duct betweenthe boiler 11 and the air heater 13. Accordingly, an amount ofdesulfurized water to be returned to the boiler 11 and the inside of theflue gas duct D per unit of time can be increased as compared toconventional systems.

The NO_(x) removal equipment 12 removes nitrogen oxides in the flue gas17 from the boiler 11, and includes an NO_(x) removal catalyst layer(not shown) therein. A reducing agent injector (not shown) is arrangedon an upstream side of the NO_(x) removal catalyst layer, and a reducingagent is injected to the flue gas 17 from the reducing agent injector.As the reducing agent, for example, ammonia, urea, or ammonium chlorideis used. Flue gas 17 introduced into the NO_(x) removal equipment 12comes in contact with the NO_(x) removal catalyst layer, and nitrogenoxides in the flue gas 17 are decomposed into nitrogen gas (N₂) andwater (H₂O) and removed. When a chlorine (Cl) content in the flue gas 17increases, the proportion of a bivalent mercury chloride soluble inwater increases, and mercury can be easily collected by the desulfurizer15 described later.

The NO_(x) removal equipment 12 is not essential, and when theconcentration of nitrogen oxides or the mercury concentration in theflue gas 17 from the boiler 11 is very low or these substances are notcontained in the flue gas 17, the NO_(x) removal equipment 12 can beomitted.

The air heater 13 is a heat exchanger that recovers heat in the flue gas17 in which nitrogen oxides have been removed by the NO_(x) removalequipment 12. Because the temperature of the flue gas 17 having passedthrough the NO_(x) removal equipment 12 is as high as about 350° C. to400° C., the air heater 13 performs heat exchange between thehigh-temperature flue gas 17 and combustion air at a normal temperature.Combustion air, which becomes high temperature by heat exchange, issupplied to the boiler 11. On the other hand, the flue gas 17 havingbeen heat-exchanged with combustion air at a normal temperature iscooled to about 150° C.

The precipitator 14 reduces dust in the flue gas 17 after heat recovery.As the precipitator 14, a centrifugal precipitator, a filteringprecipitator, and an electric precipitator can be mentioned; however, itis not particularly limited thereto.

The desulfurizer 15 reduces sulfur oxides in the flue gas 17 after dustreduction according to a wet method.

In the desulfurizer 15, a limestone slurry 20 (a solution in whichlimestone powder is dissolved in water) is used as an alkalineabsorbent, and the temperature inside the desulfurizer is adjusted toabout 30° C. to 50° C. The limestone slurry 20 is supplied from alimestone-slurry supply system 21 to a column bottom part 22 of thedesulfurizer 15. The limestone slurry 20 supplied to the column bottompart 22 of the desulfurizer 15 is supplied to a plurality of nozzles 23in the desulfurizer 15 via an absorbent supply line (not shown), and isejected from the nozzles 23 toward a column top part 24 of thedesulfurizer 15. Because the flue gas 17 rising from the column bottompart 22 of the desulfurizer 15 comes in gas-liquid contact with thelimestone slurry 20 ejected from the nozzles 23, sulfur oxides andmercury chloride in the flue gas 17 are absorbed by the limestone slurry20, and separated and removed from the flue gas 17. The flue gas 17purified by the limestone slurry 20 is discharged from the column toppart 24 of the desulfurizer 15 as purged gas 26, and discharged tooutside of the air pollution control system from a stack 27.

At the inside of the desulfurizer 15, sulfur oxides SO_(x) in the fluegas 17 causes a reaction with a limestone slurry 19 represented by thefollowing expression (1).

CaCO₃+SO₂+0.5H₂O→CaSO₃·0.5H₂O+CO₂   (1)

The limestone slurry 20 that has absorbed SO_(x) in the flue gas 17 isthen oxidized by air (not shown) supplied to the column bottom part 22of the desulfurizer 15, to cause a reaction with air represented by thefollowing expression (2).

CaSO₃·0.5H₂O+0.5O₂+1.5H₂O→CaSO₄·2H₂O   (2)

In this manner, SO_(x) in the flue gas 17 is captured in a state ofgypsum CaSO₄·2H₂O in the desulfurizer 15.

As described above, while a solution accumulated in the column bottompart 22 of the desulfurizer 15 and pumped is used as the limestoneslurry 20, gypsum CaSO₄·2H₂O is mixed in the limestone slurry to bepumped by an operation of the desulfurizer 15, according to the abovereaction expressions (1) and (2). The limestone gypsum slurry (alimestone slurry mixed with gypsum) to be pumped is hereinafter referredto as “absorbent”.

The absorbent (the limestone gypsum slurry) used for desulfurization isdischarged to outside from the column bottom part 22 of the desulfurizer15 as the desulfurized waste water 28, and supplied to a waste watertank 31 via a desulfurized waste-water line 29 explained below. As wellas gypsum, heavy metal such as mercury and halogen ions such as Cl⁻,Br⁻, I⁻, and F⁻ are included in the desulfurized waste water 28.

The boiler 11, the NO_(x) removal equipment 12, the air heater 13, theprecipitator 14, and the desulfurizer 15 are connected by one flue gasduct D. A bypass pipe 32 for connecting the flue gas duct D on anupstream side and on a downstream side of the NO_(x) removal equipment12 is provided at a position parallel to the NO_(x) removal equipment12. Likewise, a bypass pipe 33 for connecting the flue gas duct D on anupstream side and on a downstream side of the air heater 13 is providedat a position parallel to the air heater 13. By having such aconfiguration, the desulfurized waste water 28 can be atomized also intothe flue gas 17 circulating in the respective bypass pipes 32 and 33.The respective bypass pipes 32 and 33 are configured so that an amountof flue gas circulating therein becomes about several percents of theamount of flue gas circulating in the flue gas duct D.

The waste-water atomizing device 16 includes the waste water line 29 forsupplying the desulfurized waste water (gypsum slurry) 28 dischargedfrom the desulfurizer 15 to the waste water tank 31, the waste watertank 31 that accumulates desulfurized waste water 28, and a plurality ofwaste-water supplying pipes (waste-water supplying units) P0 to P5respectively connected to the waste water tank 31 to supply thedesulfurized waste water 28 accumulated in the waste water tank 31 intothe path for supplying the fuel F to the boiler 11, the inside of afurnace of the boiler 11, the inside of the flue gas duct D, and theinside of the bypass pipes 32 and 33. Nozzles N1 to N5 for atomizing thedesulfurized waste water 28 are fitted to the ends of the waste-watersupplying pipes P1 to P5.

The waste-water supplying pipes P1 to P5 are installed at positionswhere the high-temperature flue gas 17 before heat recovery by the airheater 13 is circulated, that is, on the upstream side of the air heater13. In the example shown in FIG. 1, the waste-water supplying pipe P1 isconnected to the boiler 11, and the nozzle N1 is installed at the insideof a furnace of the boiler 11. Specifically, the nozzle N1 is installedon the side of the furnace or on a furnace wall in an upper part of thefurnace, so that the desulfurized waste water 28 is atomized from thenozzle N1 toward a flame portion at the center of the furnace or abovethe flame. The waste-water supplying pipe P2 is connected to the fluegas duct D between an outlet of the boiler 11 and the NO_(x) removalequipment 12, and the nozzle N2 is installed to the inside of the fluegas duct D. The waste-water supplying pipe P3 is connected to the bypasspipe 32 for connecting the flue gas ducts D on the upstream side and onthe downstream side of the NO_(x) removal equipment 12, and the nozzleN3 is installed to the inside of the bypass pipe 32. The waste-watersupplying pipe P4 is connected to the flue gas duct D between the NO_(x)removal equipment 12 and the air heater 13, and the nozzle N4 isinstalled to the inside of the flue gas duct D. The waste-watersupplying pipe P5 is connected to the bypass pipe 33 for connecting theflue gas ducts D on the upstream side and on the downstream side of theair heater 13, and the nozzle N5 is installed to the inside of thebypass pipe 33.

As the nozzles N1 to N5, for example, a two-fluid nozzle or a rotaryatomizer is used. It is desired that a mist diameter of the nozzles N1to N5 is such that a maximum particle diameter is equal to or less than200 micrometers and an average particle diameter is from 30 to 70micrometers. Accordingly, contact efficiency with the flue gas 17 isimproved, thereby enabling to improve evaporation efficiency.

The gas temperature in a furnace of the boiler 11 where the nozzle N1 isinstalled is as high as 1400° C. to 1600° C., which is the highesttemperature in the system, and thus a largest amount of the desulfurizedwaste water 28 can be evaporated. Further, the flue gas temperatureinside the flue gas duct D between the outlet of the boiler 11 and theNO_(x) removal equipment 12 where the nozzle N2 is installed is about500° C., and the flue gas temperature in the flue gas duct D between theNO_(x) removal equipment 12 and the air heater 13 where the nozzle N4 isinstalled and in the bypass pipes 32 and 33 where the nozzles N3 and N5are installed is respectively about 350° C. to 400° C., and although thetemperature is lower than that in the furnace of the boiler 11, thedesulfurized waste water 28 can be reliably evaporated. On the otherhand, the temperature of the flue gas 17 having passed through the airheater 13 decreases to about 150° C., and thus the desulfurized wastewater 28 cannot be evaporated sufficiently.

The waste-water supplying pipe P0 is installed in the path for supplyingthe fuel F to the boiler 11. The path for supplying the fuel F to theboiler 11 is, specifically, inside of a fuel supply system (not shown)or a pipe for connecting the fuel supply system and the boiler 11. Thedesulfurized waste water 28 supplied from the waste-water supplying pipeP0 into the fuel F is mixed with the fuel F, input to the boiler 11together with the fuel F, and evaporated in the boiler.

Opening/closing valves V0 to V5 are respectively installed in thewaste-water supplying pipes P0 to P5, and by controlling anopening/closing degree of the opening/closing valves V0 to V5, a flowrate of the desulfurized waste water 28 to be supplied to thewaste-water supplying pipes P0 to P5 is adjusted. The desulfurized wastewater 28 accumulated in the waste water tank 31 passes through thewaste-water supplying pipes P1 to P5 and is atomized from the respectivenozzles N1 to N5 into the high-temperature flue gas 17 inside of thefurnace of the boiler 11, inside of the flue gas duct D, and inside ofthe bypass pipes 32 and 33, and is also supplied to the path forsupplying the fuel F to the boiler 11 through the waste-water supplyingpipe P0.

The desulfurized waste water 28 atomized from the nozzles N1 to N5 intothe high-temperature flue gas 17 is evaporated to become water vapor,and thereafter, supplied into the desulfurizer 15 together with the fluegas 17. Because the temperature in the desulfurizer 15 is as high as 30°C. to 50° C., most of the water vapor introduced into the desulfurizer15 is devolatilized, and mixed with the limestone slurry 20 in thecolumn bottom part 22. Meanwhile, the water vapor, which is notdevolatilized, is discharged from the stack 27 together with the purgedgas 26.

As described above, because the desulfurized waste water 28 is directlyatomized into the high-temperature flue gas 17 before heat recovery bythe air heater 13, even if the amount of the desulfurized waste water 28to be atomized is large, the desulfurized waste water 28 can beevaporated reliably, and thus the amount of the desulfurized waste water28 to be returned to the flue gas duct D per unit time can be increasedas compared to conventional systems. As a result, the whole amount ofthe desulfurized waste water 28 discharged from the desulfurizer 15 canbe returned to the flue gas duct D, and discharge of waste water to theoutside of the system can be eliminated completely.

Because an amount of the desulfurized waste water 28 to be dischargedfrom the desulfurizer 15 also increases in proportion to the treatingamount of the flue gas 17, the amount of the desulfurized waste water 28to be returned to the flue gas duct D per unit time is increased,thereby increasing an amount of waste water that can be treated per unittime. As a result, the treating amount of flue gas per unit time can beincreased as compared to conventional systems.

In the first embodiment, the bypass pipe 32 that connects the flue gasducts D on the upstream side and on the downstream side of the NO_(x)removal equipment 12 and the bypass pipe 33 that connects the flue gasducts D on the upstream side and on the downstream side of the airheater 13 are provided, and the desulfurized waste water 28 is atomizedinto the flue gas 17 inside the bypass pipes 32 and 33 and evaporated.Therefore, when there is a possibility that dry particles such as ashgenerated due to evaporation of the desulfurized waste water 28 passthrough the NO_(x) removal equipment 12 and the air heater 13 todecrease effects of these devices, dry particles can be efficientlydelivered to the downstream side of the NO_(x) removal equipment 12 andthe air heater 13 via the respective bypass pipes 32 and 33.

As described above, the temperature of the flue gas 17 is differentaccording to positions in the flue gas duct D, and evaporationefficiency of the desulfurized waste water 28 is also different.Therefore, the opening/closing degree of the valves V0 to V5 isoptimized, taking into consideration delivery efficiency of dryparticles and evaporation efficiency of the flue gas 17.

The configuration of the waste-water atomizing device 16 shown in FIG. 1is only an example, and the number of installation and installationpositions of the waste-water supplying pipes P0 to P5 are not limitedthereto, and can be appropriately changed according to the amount of thedesulfurized waste water 28 and the kind of the flue gas 17. That is,the waste-water supplying pipes P0 to P5 only need to be installed atleast at one position of the path for supplying the fuel F to the boiler11, the inside of a furnace of the boiler 11, the flue gas duct D froman outlet of the boiler 11 to an inlet of the air heater, and the bypasspipes 32 and 33.

When harmful substances and solid contents are contained in thedesulfurized waste water 28 only in a small amount, and even if thedesulfurized waste water 28 is atomized into the flue gas duct D on theupstream side of the NO_(x) removal equipment 12 and the air heater 13,there is no possibility of decreasing effects of the NO_(x) removalequipment 12 and the air heater 13, the bypass pipes 32 and 33 do notneed to be provided. Further, it is not required to provide both of thebypass pipes 32 and 33, and only either one of bypass pipes can beprovided.

Further, in the waste-water atomizing device 16 shown in FIG. 1, thedesulfurized waste water 28 is temporarily accumulated in the wastewater tank 31, and the desulfurized waste water 28 is supplied from thewaste water tank 31 to the waste-water supplying pipes P0 to P5.However, the desulfurized waste water 28 supplied from the desulfurizer15 can be directly supplied to the waste-water supplying pipes P0 to P5.

As explained above, in the air pollution control system 10A according tothe first embodiment, the waste-water supplying pipes P0 to P5 thatsupply the desulfurized waste water 28 discharged from the desulfurizer15 to at least one of the path for supplying the fuel F to the boiler11, the inside of a furnace of the boiler 11, and the inside of the fluegas duct D from the boiler 11 to the air heater 13 are installed, andthe desulfurized waste water 28 is directly supplied by the waste-watersupplying pipes P0 to P5. By having such a configuration, because adevice for gasifying desulfurized waste water does not need to beprovided separately like in conventional systems, the amount of thedesulfurized waste water 28 to be discharged to the outside of thesystem can be decreased, without increasing the size of the entire airpollution control system.

Further, because the desulfurized waste water 28 is directly atomizedinto the high-temperature flue gas 17 before heat recovery by the airheater 13, even if the amount of the desulfurized waste water 28 to beatomized is large, the desulfurized waste water 28 can be evaporatedreliably, and the amount of the desulfurized waste water 28 to bereturned to the flue gas duct D per unit time can be increased ascompared to conventional systems. As a result, the whole amount of thedesulfurized waste water 28 to be discharged from the desulfurizer 15can be returned to the flue gas duct D, and discharge of waste water tothe outside of the system can be eliminated completely. Further, theamount of the desulfurized waste water 28 to be returned to the flue gasduct D per unit time is increased as compared to conventional systems,thereby increasing an amount of the desulfurized waste water 28 that canbe treated per unit time. As a result, the treating amount of flue gasper unit time can be increased as compared to conventional systems.

The bypass pipe 32 that connects the flue gas ducts D on the upstreamside and on the downstream side of the NO_(x) removal equipment 12 isprovided, and the desulfurized waste water 28 is atomized into the fluegas 17 inside the bypass pipe 32 and evaporated. Therefore, dryparticles such as ash generated due to evaporation of the desulfurizedwaste water 28 can be efficiently delivered to the downstream side ofthe NO_(x) removal equipment 12 via the bypass pipe 32. Likewise, thebypass pipe 33 that connects the flue gas ducts D on the upstream sideand on the downstream side of the air heater 13 is provided, and thedesulfurized waste water 28 is atomized into the flue gas 17 inside thebypass pipe 33 and evaporated. Therefore, dry particles such as ashgenerated due to evaporation of the desulfurized waste water 28 can beefficiently delivered to the downstream side of the air heater 13 viathe bypass pipe 33.

In the example shown in FIG. 1, a case that the whole amount of thedesulfurized waste water 28 discharged from the desulfurizer 15 isreturned to at least one of the path for supplying the fuel F to theboiler 11, the inside of a furnace of the boiler 11, the inside of theflue gas duct D, and the inside of the bypass pipes 32 and 33 isexplained. However, when the amount of the desulfurized waste water 28increases due to an increase in the treating amount of the flue gas 17and the whole amount of the desulfurized waste water 28 cannot bereturned, after harmful substances are removed and pH is adjusted, apart of the desulfurized waste water 28 can be discharged to the outsideof the system.

Second Embodiment

An air pollution control system according to a second embodiment isexplained next. Constituent elements identical to those in the firstembodiment described above are denoted by like reference signs andexplanations thereof will be omitted. FIG. 2 is a schematicconfiguration diagram of an air pollution control system 10B accordingto the second embodiment. In the first embodiment, the desulfurizedwaste water 28 is directly atomized into the flue gas 17 in the boiler11 and inside of the flue gas duct D without performing the waste-watertreatment of the desulfurized waste water 28 discharged from thedesulfurizer 15. However, in the air pollution control system 10Baccording to the second embodiment, it is different from the firstembodiment that a solid-liquid separating device 35 is provided in themiddle of the waste water line 29 so that the desulfurized waste water28 from the desulfurizer 15 is separated into a solid fraction and aliquid fraction by the solid-liquid separating device 35, and theseparate liquid (treated waste water) is supplied into the path forsupplying the fuel F to the boiler 11, the inside of a furnace of theboiler 11, the inside of the flue gas duct D, and the inside of thebypass pipes 32 and 33. Other configurations of the second embodimentare identical to those of the first embodiment.

The solid-liquid separating device 35 separates the desulfurized wastewater 28 into a solid fraction including gypsum and a liquid fraction.As the solid-liquid separating device 35, for example, a belt filter, acentrifugal separator, or a decanter type centrifugal settler is used.Gypsum 37 in the desulfurized waste water 28 discharged from thedesulfurizer 15 is separated by the solid-liquid separating device 35.At this time, mercury chloride in the desulfurized waste water 28 isseparated from the liquid together with the gypsum 37 in a state ofbeing adsorbed on the gypsum 37. The separated gypsum 37 is dischargedto outside of the air pollution control system (hereinafter, “outside ofthe system”). On the other hand, separate liquid (treated waste water)36 is supplied to the waste water tank 31 via the waste water line 29.The separate liquid (treated waster water) 36 accumulated in the wastewater tank 31 is supplied into the path for supplying the fuel F to theboiler 11, the inside of a furnace of the boiler 11, the inside of theflue gas duct D, and the inside of the bypass pipes 32 and 33 via thewaste-water supplying pipes P0 to P5, and evaporated.

As described above, in the air pollution control system 10B according tothe second embodiment, the gypsum 37 is separated from the desulfurizedwaste water 28 discharged from the desulfurizer 15, and the separateliquid 36 is supplied into the path for supplying the fuel F to theboiler 11, the inside of a furnace of the boiler 11, the inside of theflue gas duct D, and the inside of the bypass pipes 32 and 33. By havingsuch a configuration, the amount of dry particles generated due toevaporation of waste water in the flue gas duct D can be decreased ascompared to the first embodiment, in addition to effects of the firstembodiment. As a result, reaction inhibition in the NO_(x) removalequipment 12 due to adhesion of dry particles and clogging of the NO_(x)removal equipment 12 and the air heater 13 can be decreased, therebyimproving the flexibility of installation positions of the nozzles N1 toN5. Further, because mercury chloride is separated and removed togetherwith the gypsum 37 by solid-liquid separation of the desulfurized wastewater 28, an increase in mercury concentration in the flue gas 17 insidethe flue gas duct D can be prevented at the time of atomizing wastewater.

Third Embodiment

An air pollution control system according to a third embodiment isexplained next. Constituent elements identical to those in the first andsecond embodiments described above are denoted by like reference signsand explanations thereof will be omitted. FIG. 3 is a schematicconfiguration diagram of the air pollution control system 10B accordingto the second embodiment. In the second embodiment, the solid-liquidseparating device 35 is provided in the middle of the waste water line29 to perform solid-liquid separation of the desulfurized waste water 28discharged from the desulfurizer 15, and the separate liquid 36 issupplied into the path for supplying the fuel F to the boiler 11, theinside of a furnace of the boiler 11, the inside of the flue gas duct D,and the inside of the bypass pipes 32 and 33. However, in an airpollution control system 10C according to the third embodiment, it isdifferent from the second embodiment that a waste-water treatment device38 is further provided on a downstream side of the solid-liquidseparating device 35, and treated waste water 39 is atomized into theflue gas 17 after harmful substances, suspended solids or the like inthe separate liquid 36 are removed by the waste-water treatment device38. Other configurations of the third embodiment are identical to thoseof the second embodiment.

The waste-water treatment device 38 includes a unit that removessubstances such as mercury (that cannot be adsorbed on the gypsum 37),boron, and selenium remaining in the separate liquid 36 (hereinafter,“mercury removing unit”), and a unit that removes halogen ions such aschlorine ion (C1 ⁻), bromine ion (Br⁻), iodine ion (I⁻), and fluorineion (F⁻) (hereinafter, “halogen-ion removing unit”).

Substances such as mercury, boron, and selenium are easily dissolved inwater and volatilize at the time of being atomized into the flue gas 17,and thus it is difficult to remove these substances by the precipitator14. As the means for removing these substances, removal by precipitationdue to cohesion by adding a sulfide coagulation aid, removal byadsorption (an entrained bed) on activated carbon, removal byprecipitation by adding a chelating agent, and crystallization can bementioned. The harmful substances are solidified by the mercury removingunit described above, and solids are discharged to the outside of thesystem.

Because halogen ions have a property of suppressing adsorption ofmercury on the gypsum 37 in a desulfurizing process performed by thedesulfurizer 15, it is desired to remove halogen ions from thedesulfurized waste water 28. As the unit that removes halogen ions, aconcentrating unit using a reverse osmosis membrane, a concentratingunit using an ion exchange membrane, a concentrating unit usingelectrodialysis can be mentioned, as well as a distillation orcrystallization technique. Halogen ions are concentrated by thehalogen-ion removing unit described above, and concentrates aredischarged to the outside of the system.

The gypsum 37 that has adsorbed mercury chloride is separated from thedesulfurized waste water 28 discharged from the desulfurizer 15 by thesolid-liquid separating device 35, and the gypsum 37 is discharged tothe outside of the system. The separate liquid 36 in which the gypsum 37has been removed is supplied to the waste-water treatment device 38 viathe waste water line 29, and harmful substances such as mercury, boron,and selenium remaining in the separate liquid 36 is removed by themercury removing unit. The treated waste water after mercury has beenremoved is supplied to the halogen-ion removing unit, where halogen ionsare removed. The treated waste water 39 after halogen ions have beenremoved is supplied to the waste water tank 31. The treated waste water39 accumulated in the waste water tank 31 is supplied into the path forsupplying the fuel F to the boiler 11, the inside of a furnace of theboiler 11, the inside of the flue gas duct D, and the inside of thebypass pipes 32 and 33 via the waste-water supplying pipes P0 to P5, andevaporated.

The waste-water treatment device 38 does not need to include both of themercury removing unit and the halogen-ion removing unit, and either unitis selected according to the property of the desulfurized waste water 28and installed. For example, when treatment for oxidizing mercury by theNO_(x) removal equipment 12 and converting oxidized mercury to mercurychloride is to be performed, because Cl⁻ can be useful, the treatmentperformed by the halogen-ion removing unit can be omitted, and thetreated waste water 39 containing halogen ions can be atomized into theflue gas 17. When mercury is sufficiently removed in the solid-liquidseparating device 35 on the upstream side of the waste-water treatmentdevice 38, and a mercury content is considerably low or mercury is notcontained in the separate liquid 36, the treatment performed by themercury removing unit can be omitted.

The order of the mercury removal treatment and halogen-ion removaltreatment performed by the waste-water treatment device 38 is notparticularly limited. That is, the halogen-ion removal treatment can beperformed after performing the mercury removal treatment, or the mercuryremoval treatment can be performed after performing the halogen-ionremoval treatment.

As described above, in the air pollution control system 10C according tothe third embodiment, after the gypsum 37, which is a large object, isseparated from the desulfurized waste water 28 discharged from thedesulfurizer 15, fine substances such as mercury, boron, selenium, andhalogen ions are removed, and the treated waste water 39 is suppliedinto the path for supplying the fuel F to the boiler 11, the inside of afurnace of the boiler 11, the inside of the flue gas duct D, and theinside of the bypass pipes 32 and 33. By having such a configuration, anincrease in mercury concentration in the flue gas 17 inside of the fluegas duct D can be prevented at the time of atomizing waste water, inaddition to effects of the second embodiment that an amount of dryparticles generated due to evaporation of waste water in the flue gasduct D can be decreased.

Fourth Embodiment

An air pollution control system according to a fourth embodiment isexplained next. Constituent elements identical to those in the first tothird embodiments described above are denoted by like reference signsand explanations thereof will be omitted. FIG. 4 is a schematicconfiguration diagram of an air pollution control system 10D accordingto the fourth embodiment. In the third embodiment, the waste-watertreatment device 38 is provided on the downstream side of thesolid-liquid separating device 35. However, in the fourth embodiment, itis different from the third embodiment that the waste-water treatmentdevice 38 is provided on the upstream side of the solid-liquidseparating device 35. Other configurations of the fourth embodiment areidentical to those of the third embodiment.

The desulfurized waste water 28 discharged from the desulfurizer 15 isfirst supplied to the waste-water treatment device 38, where finesubstances such as mercury, boron, and selenium contained in thedesulfurized waste water 28 are solidified by the means using cohesion,adsorption on activated carbon, chelating agent, or crystallizationdescribed above. Further, halogen ions such as Cl⁻, Br⁻, I⁻, and F⁻become concentrated by the concentration means using the reverse osmosismembrane, ion exchange membrane, or electrodialysis, distillation or thelike as described above, and concentrates are separated. Treated wastewater 41 containing solid substances such as mercury is supplied to thesolid-liquid separating device 35 via the waste water line 29, and thesolid substances are separated and removed together with the gypsum 37containing mercury chloride. Separate liquid 42 separated from the solidcontent by the solid-liquid separating device 35 is supplied to thewaste water tank 31. The separate liquid (treated waste water) 42accumulated in the waste water tank 31 is supplied into the path forsupplying the fuel F to the boiler 11, the inside of a furnace of theboiler 11, the inside of the flue gas duct D, and the inside of thebypass pipes 32 and 33 via the waste-water supplying pipes P0 to P5, andevaporated.

In this manner, in the air pollution control system 10D according to thefourth embodiment, fine substances such as mercury, boron, selenium, andhalogen ions contained in the desulfurized waste water 28 dischargedfrom the desulfurizer 15 are solidified, the solid substance isseparated together with the gypsum 37 by the solid-liquid separatingdevice 35, and the separate liquid 42 is supplied into the path forsupplying the fuel F to the boiler 11, the inside of a furnace of theboiler 11, the inside of the flue gas duct D, and the inside of thebypass pipes 32 and 33. By having such a configuration, an increase inmercury concentration in the flue gas 17 inside of the flue gas duct Dcan be prevented at the time of atomizing waste water, in addition toeffects of the second embodiment that an amount of dry particlesgenerated due to evaporation of waste water can be decreased. The solidsubstances and concentrates generated in the waste-water treatmentdevice 38 are separated and removed together with the gypsum 37 by thesolid-liquid separating device 35 on the downstream side, and thus afiltering process in the waste-water treatment device 38 can be omitted.

Fifth Embodiment

An air pollution control system according to a fifth embodiment isexplained next. Constituent elements identical to those in the first tofourth embodiments described above are denoted by like reference signsand explanations thereof will be omitted. FIG. 5 is a schematicconfiguration diagram of an air pollution control system 10E accordingto the fifth embodiment. In the fifth embodiment, a second waste-watertreatment device 38B is further provided on the downstream side of thesolid-liquid separating device 35, in addition to the configuration ofthe fourth embodiment. The configuration of the second waste-watertreatment device 38B is the same as that of a first waste-watertreatment device 38A installed on the upstream side of the solid-liquidseparating device 35.

The desulfurized waste water 28 discharged from the desulfurizer 15 isfirst supplied to the first waste-water treatment device 38A, where finesubstances such as mercury, boron, and selenium contained in thedesulfurized waste water 28 are solidified by the means of cohesion,adsorption on activated carbon, chelating agent, or crystallizationdescribed above. Halogen ions such as Cl⁻, Br⁻, I⁻, and F⁻ becomeconcentrated by the concentration means using the reverse osmosismembrane, ion exchange membrane, or electrodialysis, or distillationdescribed above. The treated waste water 41 containing solid substancessuch as mercury and concentrates of halogen ions is supplied to thesolid-liquid separating device 35 via the waste water line 29, and thesolid substances and the concentrates are separated and removed togetherwith the gypsum 37 containing mercury chloride. The separate liquid 42separated by the solid-liquid separating device 35 is supplied to thesecond waste-water treatment device 38B, where a small amount of mercuryand halogen ions remaining in the separate liquid 42 are removed.Treated waste water 43 treated by the second waste-water treatmentdevice 38B is supplied to the waste water tank 31. The treated wastewater 42 accumulated in the waste water tank 31 is supplied into thepath for supplying the fuel F to the boiler 11, the inside of a furnaceof the boiler 11, the inside of the flue gas duct D, and the inside ofthe bypass pipes 32 and 33 via the waste-water supplying pipes P0 to P5,and evaporated.

As described above, in the air pollution control system 10E according tothe fifth embodiment, after fine substances such as mercury, boron,selenium, and halogen ions in the desulfurized waste water 28 dischargedfrom the desulfurizer 15 are solidified by the first waste-watertreatment device 38A, the solid substances are separated together withthe gypsum 37 by the solid-liquid separating device 35. The treatedwaste water 43 after a small amount of mercury and halogen ionsremaining in the separate liquid 42 are removed by the secondwaste-water treatment device 38B is then supplied into the path forsupplying the fuel F to the boiler 11, the inside of a furnace of theboiler 11, the inside of the flue gas duct D, and the inside of thebypass pipes 32 and 33. By performing waste water treatment highlyaccurately in this manner, dry particles are hardly generated when thetreated waste water 43 is atomized into the flue gas 17 and evaporated,and an increase in mercury concentration in the flue gas 17 inside theflue gas duct D can be prevented reliably at the time of atomizing wastewater.

INDUSTRIAL APPLICABILITY

As described above, the air pollution control system and the airpollution control method according to the present invention are usefulfor decreasing desulfurized waste water discharged from a desulfurizeror completely eliminating discharge of desulfurized waste water tooutside.

Reference Signs List

-   10A, 10 b, 10C, 10D, 10E air pollution control system-   11 boiler-   12 NO_(x) removal equipment-   13 air heater-   14 precipitator-   15 desulfurizer-   16 waste-water atomizing device-   17 flue gas-   20 limestone slurry-   21 limestone-slurry supply system-   22 column bottom part-   23 nozzle-   24 column top part-   26 purged gas-   27 stack-   28 desulfurized waste water-   29 waste water line-   31 waste water tank-   32 bypass pipe-   33 bypass pipe-   35 solid-liquid separating unit-   36, 42 separate liquid-   37 gypsum-   38 waste-water treatment device-   39, 41, 43 treated waste water-   P0, P1, P2, P3, P4, P5 waste-water supply pipe (waste-water    supplying unit)-   N1, N2, N3, N4, N5 nozzle-   F fuel

1. An air pollution control system comprising: a boiler that burns fuel;an air heater that recovers heat of flue gas from the boiler; adesulfurizer that reduces sulfur oxides contained in flue gas after heatrecovery by an absorbent; a waste-water supplying unit that suppliesdesulfurized waste water discharged from the desulfurizer to at leastone of a path for supplying fuel to the boiler, inside of a furnace ofthe boiler, and inside of a flue between the boiler and the air heater.2. The air pollution control system according to claim 1, wherein aNO_(x) removal equipment that removes nitrogen oxides in flue gas fromthe boiler is provided on an upstream side of the air heater, and thewaste-water supplying unit is provided at least at one position betweenthe boiler and the NO_(x) removal equipment or between the NO_(x)removal equipment and the air heater.
 3. The air pollution controlsystem according to claim 2, wherein a bypass pipe is provided at leastat one position parallel to the NOx removal equipment or parallel to theair heater, and the waste-water supplying unit is provided in the bypasspipe.
 4. An air pollution control system comprising: a boiler that burnsfuel; an air heater that recovers heat of flue gas from the boiler; adesulfurizer that reduces sulfur oxides contained in flue gas after heatrecovery by an absorbent; a waste-water treating unit that removesharmful substances from desulfurized waste water discharged from thedesulfurizer; and a waste-water supplying unit installed at least at oneposition in a path for supplying fuel to the boiler, inside of a furnaceof the boiler, or inside of a flue between the boiler and the air heaterto atomize treated waste water treated by the waste-water treating unit.5. The air pollution control system according to claim 4, wherein aNO_(x) removal equipment that removes nitrogen oxides in flue gas fromthe boiler is provided on an upstream side of the air heater, and thewaste-water supplying unit is provided at least at one position betweenthe boiler and the NO_(x) removal equipment or between the NO_(x)removal equipment and the air heater.
 6. The air pollution controlsystem according to claim 5, wherein a bypass pipe is provided at leastat one position parallel to the NOx removal equipment or parallel to theair heater, and the waste-water supplying unit is provided in the bypasspipe.
 7. The air pollution control system according to claim 4, whereinthe waste-water treating unit includes a solid-liquid separating devicethat separates desulfurized waste water discharged from the desulfurizerinto a solid and a liquid.
 8. The air pollution control system accordingto claim 4, wherein the waste-water treating unit includes a mercuryremoving device that removes mercury contained in desulfurized wastewater discharged from the desulfurizer.
 9. The air pollution controlsystem according to claim 4, wherein the waste-water treating unitincludes a halogen-ion removing device that removes halogen ionscontained in desulfurized waste water discharged from the desulfurizer.10. An air pollution control method in which after heat of flue gas froma boiler that burns fuel is recovered by an air heater, a desulfurizerreduces sulfur oxides contained in flue gas after heat recovery by anabsorbent, wherein desulfurized waste water discharged from thedesulfurizer is supplied to at least one of a path for supplying fuel tothe boiler, inside of a furnace of the boiler, and inside of a flue gasduct between the boiler and the air heater.
 11. The air pollutioncontrol method according to claim 10, wherein a NO_(x) removal equipmentthat removes nitrogen oxides in flue gas from the boiler is provided onan upstream side of the air heater, and the desulfurized waste water issupplied to at least one position between the boiler and the NO_(x)removal equipment or between the NO_(x) removal equipment and the airheater.
 12. The air pollution control method according to claim 11,wherein a bypass pipe is provided at least at one position parallel tothe NO_(x) removal equipment or parallel to the air heater, and thedesulfurized waste water is supplied to inside of the bypass pipe. 13.An air pollution control method in which after heat of flue gas from aboiler that burns fuel is recovered by an air heater, a desulfurizerreduces sulfur oxides contained in flue gas after heat recovery by anabsorbent, wherein after waste water treatment is performed for removingharmful substances in desulfurized waste water discharged from thedesulfurizer, treated waste water treated in the waste water treatmentis supplied to at least one of a path for supplying fuel to the boiler,inside of a furnace of the boiler, and inside of a flue gas duct betweenthe boiler and the air heater.
 14. The air pollution control methodaccording to claim 13, wherein a NO_(x) removal equipment that removesnitrogen oxides in flue gas from the boiler is provided on an upstreamside of the air heater, and the treated waste water is supplied to atleast one position between the boiler and the NO_(x) removal equipmentor between the NO_(x) removal equipment and the air heater.
 15. The airpollution control method according to claim 14, wherein a bypass pipe isprovided at least at one position parallel to the NO_(x) removalequipment or parallel to the air heater, and the treated waste water issupplied to inside of the bypass pipe.
 16. The air pollution controlmethod according to claim 13, wherein the waste water treatment includesa solid-liquid separating step of separating desulfurized waste waterdischarged from the desulfurizer into a solid and a liquid.
 17. The airpollution control method according to claim 13, wherein the waste watertreatment includes a mercury removing step of removing mercury containedin desulfurized waste water discharged from the desulfurizer.
 18. Theair pollution control method according to claim 13, wherein the wastewater treatment includes a halogen-ion removing step of removing halogenions contained in desulfurized waste water discharged from thedesulfurizer.